The present invention relates generally to integrated circuits for microwave devices. More specifically, the present invention relates to a microwave oscillator using a negative resistance cell integrated on a monolithic substrate using BiCMOS fabrication techniques.
Negative resistance cells are well known in the art for use in an active circuit to provide an oscillating system for generation of periodic output signals. FIG. 5 is a block diagram or a conventional negative resistance oscillator 100 having a resonance circuit 102 coupled to an active circuit 104. The combination of the resonance circuit 102 and the active circuit 104 produce an oscillator. Gonzalez, Microwave Transistor Amplifiers, Prentice Hall, 1989, chapter 5, pages 194-199 describe microwave transistor oscillator design and operation in more detail, and is hereby expressly incorporated by reference for all purposes.
FIG. 6 is a block diagram of oscillator 100 showing further detail. Resonance circuit 102 includes an impedance 110, an inductance 112, and a capacitance 114, all coupled in parallel between an output port 116 and ground 118. Active circuit 104 includes a negative resistance cell 120 connected to output port 116, ground 118, and an output circuit 122. Output circuit 122 includes a series connected capacitance 124 and a load resistance 126 that couples negative resistance cell 120 to RF output port 130. The capacitance 124 blocks DC current.
FIG. 7 is a schematic diagram of a preferred embodiment of basic negative resistance cell 120 shown in FIG. 6. Negative resistance cell 120 includes a bipolar junction NPN transistor 150, having a parallel connected impedance 152 and a capacitance 154 coupled between its emitter and ground 118. The collector of transistor 150 connects to power. Vcc. The base of transistor 150 couples through impedance 160 to Vcc, and through impedance 162, ground 118. The base of transistor 150 couples directly to resonance circuit 102.
Oscillator 100 includes output buffer 122 coupled to the collector of transistor 150. One problem with oscillator 100 shown in FIG. 6 is that a phenomenon known as Miller capacitance existing between the collector and the base of transistor 150 couples any voltage variations at node 170 back to the base of the transistor. These voltage variations influence the oscillation frequency, and thereby affect the output of oscillator 100. To address this difficulty, conventional designs of oscillator 100 include differing buffering systems coupling the RF output to the collector of transistor 150 of the negative resistance cell 120. FIG. 8--FIG. 10 show different embodiments of the prior art to address buffering of the output signal.
FIG. 8 is a schematic diagram of one conventional approach to buffer the to output signal of oscillator 100 having a modified active circuit 104'. Active circuit 104' includes a buffer 122'. Buffer 122' includes an NPN transistor 200. and NPN transistor 202, current sources 204 and 206, and impedances 208 and 210. Buffering system 122' shown in FIG. 8 is susceptible to the Miller capacitance problem that degrades the function of oscillator 100.
FIG. 9 is a schematic diagram of one conventional approach to buffer the output signal of oscillator 100 having a modified active circuit 104". Active circuit 104" is similar to active circuit 104' and it includes an NPN transistor 220 between impedance 208 and NPN transistor 150 of negative resistance cell 120, providing a cascade configuration. The base of NPN transistor 200 is coupled to the collector of NPN transistor 220. NPN transistor 220 serves to isolate the buffer and amplifier circuitry from the base of negative resistance cell transistor 150. The system shown in FIG. 9 requires a Vcc greater than 3 volts for effective operation, and is therefore undesirable for use in circuits that are to operate at less than 3 volts, such as required for the currently emerging semiconductor standard.
FIG. 10 is a schematic diagram of one conventional approach to buffer the output signal of oscillator 100 having a modified active circuit 104'". Active circuit 104'" is similar to active circuit 104' and includes a buffer circuit 122'". The buffer 122'" includes an isolating bipolar transistor 250 having a base connected to the base of transistor 150. The collector of transistor 250 connects to Vcc, while an impedance 252 couples the emitter of transistor 250 to ground. Buffer 122'" includes a second transistor 254, for amplification. The base of transistor 252 connects to the emitter of transistor 250 and its emitter connects to ground. An impedance 256 couples Vcc to the collector of transistor 254. An impedance 258 couples the collector of transistor 254 to the RF output port.
While the configuration shown in FIG. 10 is operable and can be adjusted to function at a desired oscillation frequency, the circuit has characteristics that degrade its design and stability. This characteristic includes the observation that the capacitances associated with transistor 254, i.e. its c.sub.bc and c.sub.be parasitic capacitances, can be combined and modeled as a capacitance in parallel with impedance 252. This combination causes transistor 250 to act as a negative resistance cell. When transistor 250 acts like a negative resistance cell, its presence alters a theoretical oscillation frequency determined solely by negative resistance cell 120 and the resonance circuit 102. A design must account for more variables in order to obtain a desired oscillation frequency, resulting in an oscillator that is more difficult to control.